1. Field
The subject matter of the present invention relates to sensors. More particularly, the subject matter relates to sensors for detecting analytes.
2. Background
Conventional chemical sensors based on receptor-doped polymeric membranes (e.g., ion-selective electrodes or optical sensors) are routinely used worldwide in clinical medicine and chemistry. Every year, for example, over 200 million measurements of potassium in human blood samples are performed ex vivo with receptor-based ion-selective electrodes in hospital laboratories in the United States alone. Since also sodium, calcium, chloride, magnesium and carbonate are measured routinely in many blood samples, it is estimated that the clinical market represents over one billion measurements per year in the United States.
Conventional receptor-based chemical sensor membranes typically consist of receptors, lipophilic ions, and an inert polymer matrix. The polymer matrix is often plasticized (softened) with a compound of low molecular weight and high boiling point. The polymer matrix and plasticizer typically make up approximately 90% to 99% of the total membrane composition. Representative polymers are silicones, poly(vinyl chloride), and polyurethanes. For a review of conventional chemical sensors, see Bakker et al., Chem. Rev. 1997, 97, 3083; Bühlmann et al., Chem. Rev. 1998, 98, 1593; and Bakker et al., Talanta, 2004, 62, 843.
Unfortunately, these conventional sensors are prone to biofouling as a result of the various lipid, protein and other components present in biological samples such as blood and urine. These lipids, proteins and other components adsorbed onto, or extracted into, the sensor membranes (see Bühlmann et al., Anal. Chem. 2001, 73, 3199), causing “drifting” of the measured signal and requiring frequent replacement of the sensing membranes. Attempts have been made to reduce biofouling by counteracting surface adsorption, for example, by using membranes having modifications that cause them to release nitric oxide (Frost et al., J. Am. Chem. Soc. 2004 126(5), 1349; U.S. Pat. No. 6,841,166 (Zhang et al.; issued Jan. 11, 2005)). However, biofouling as a result of extraction or partitioning of lipids, fats, cholesterol, porphryrins and other hydrophobic and oily substances into conventional membranes remains a problem. A sensor with higher selectivity and/or reduced biofouling would represent a significant advance over existing technology for ex vivo monitoring. There is, in addition, a great need for implantable sensors (in vivo measurements) since none of the existing technologies has been shown to resist biofouling in vivo for much more than a few days.